Australia's first legally-recognised same-sex marriages have taken place in the ACT.

The WA state Labor MP Stephen Dawson and his new husband Dennis Liddelow were the first same-sex couple to make use of ACT marriage laws that came into effect on Saturday, taking the historic vows at 12.04am in a ceremony outside Parliament House.

Dawson said after the ceremony he felt "jubilant" to be married.

Up to 20 other couples from the ACT, Victoria, NSW and Queensland were due to hold their ceremonies in Canberra on Saturday, the first legally-recognised same-sex marriages in an Australian jurisdiction.

The ACT government hailed its legislation as a significant symbol of acceptance, tolerance and compassion.

But the commonwealth has already challenged the validity of the law in the high court, with a ruling due on 12 December.

"Regardless of what happens in the high court, the significance of this moment will remain and send a strong signal about what a contemporary 21st century Australia should look like," ACT attorney-general Simon Corbell said.

Australian Marriage Equality director Rodney Croome said Saturday's ceremonies were about love, commitment and family, not politics or law.

"This is an important day for same-sex couples and our families, but also for the nation, because today Australia is a fairer and more equal country that values love more highly," he said.

ACT chief minister Katy Gallagher sent a message to people who oppose same-sex marriage, saying "the world will go on".

She said the community response had been overwhelmingly positive.

"This doesn't affect those people who don't agree with it. It doesn't change their life, it doesn't change their relationships, this is all about making sure everyone is treated equally before the law," she told Sky News.

"For those people who are able to use the laws, the change is profound."

Regardless of possible opposition from the high court, Ms Gallagher said the ACT legislation has started the ball rolling.

"Just by allowing the marriages for this period of time ... the momentum is there, the world has changed, history has changed and ultimately we will see this, I'm absolutely positive, brought in across the country at some time in the future, she said."

The Australian Christian Lobby spoke against the law, forecasting "big social consequences".

"We hear about equal love all the time but we don't hear about what it means for children," Lobby spokesman Lyle Shelton told Sky News.

"Same-sex marriage means same-sex parenting, that means necessarily taking a child from its biological mother or father and giving it to someone else."